THE GENERAL SHAPES OF ANIMALS.
§ 244. Certain of the _Protozoa_ are quite indefinite in their shapes,
and quite inconstant in those indefinite shapes which they have--the
relations of their parts are indeterminate both in space and time.
In one of the simpler Rhizopods, at least during the active stage
of its existence, no permanent distinction of inside and outside is
established; and hence there can arise no established correspondence
between the shape of the outside and the distribution of environing
actions. But when the relation of inner and outer becomes fixed,
either over part of the mass or over the whole of it, we have kinds
of symmetry that correspond with the habitual incidence of forces. An
_Amœba_ in becoming encysted, passes from an indefinite, ever-changing
form into a spherical form; and the order of symmetry which it thus
assumes, is in harmony with the average equality of the actions on all
its sides. In _Difflugia_, Fig. 134, and still better in _Arcella_, we
have an indefinitely-radial symmetry occurring where the conditions are
different above and below but alike all around. Among the _Gregarinida_
the spherical symmetry and symmetry passing from that into the radial,
are such as appear to be congruous with the simple circumstances
of these creatures in the intestines of insects. But the relations
of these lowest types to their environments are comparatively so
indeterminate, and our knowledge of their actions so scanty, that
little beyond negative evidence can be expected from the study of them.
[Illustration: Figs. 131–139.]
The like may be said of the _Infusoria_. These are more or less
irregular. In some cases, where the line of movement through the
water is tolerably definite and constant, we have a form that is
approximately radial--externally at least. But usually, as shown
in Figs. 137, 138, 139, there is either an unsymmetrical or an
asymmetrical shape. And when one of these creatures is watched under
the microscope, the congruity of this shape with the incidence of
forces is manifest. For the movements are conspicuously varied and
indeterminate--movements which do not expose any two or more sides of
the mass to approximately equal sets of actions.[43]
§ 245. Among aggregates of the second order, as among aggregates of the
first order, we find that of those possessing any definite shapes the
lowest are spherical or spheroidal. Such are some of the _Radiolaria_,
as _Collozoum inerme_. These bodies which float passively in the sea,
and present in turn all their sides to the same influences, have their
parts disposed with approximate regularity round a centre--approximate,
because in the absence of locomotion a slight irregularity of
growth, almost certain to take place, may cause a fixed attitude and
a resulting deviation from spherical symmetry. The best cases in
illustration of the truth here named, are furnished by rotating and
locomotive organisms respecting which there is a dispute whether they
are animal or vegetal--the _Volvocineæ_. These, already instanced
under the one head in § 218, may here be instanced afresh under the
other. Further, among these secondary aggregates in which the units,
only physically integrated, have not had their individualities merged
into an individuality of a higher order, must be named the compound
_Infusoria_. The cluster of _Vorticellæ_ in Fig. 144, will sufficiently
exemplify them; and the striking resemblance borne by its individuals
to those of a radially-arranged cluster of flowers, will show how,
under analogous conditions, the general principles of morphological
differentiation are similarly illustrated in the two kingdoms.
§ 246. Radial symmetry is usual in low aggregates of the second order
which have their parts sufficiently differentiated and integrated
to give individualities to them as wholes. The _Cœlenterata_
offer numerous examples of this. Solitary polypes--hydroid or
helianthoid--mostly stationary, and when they move, moving with any
side foremost, do not by locomotion subject their bodies to habitual
contrasts of conditions. Seated with their mouths upwards or downwards,
or else at all degrees of inclination, the individuals of a species
taken together, are subject to no mechanical actions affecting some
parts of their discs more than other parts. And this indeterminateness
of attitude similarly prevents their relations to prey from being such
as subject some of their prehensile organs to forces unlike those to
which the rest are subject. The fixed end is differently conditioned
from the free end, and the two are therefore different; but around the
axis running from the fixed to the free end the conditions are alike
in all directions, and the form therefore is radial. Again, among many
of the simple free-swimming _Hydrozoa_, the same general truth is
exemplified under other circumstances. In a common _Medusa_, advancing
through the water by the rhythmical contractions of its disc, the
mechanical reactions are the same on all sides; and as, from accidental
causes, every part of the edge of the disc comes uppermost in its
turn, no part is permanently affected in a different way from the rest.
Hence the radial form continues.
[Illustration: Figs. 257, 258.]
In others of this same group, however, there occur forms which show
us an incipient bilateralness; and help us to see how a more decided
bilateralness may arise. Sundry of the _Medusidæ_ are proliferous,
giving origin to gemmæ from the body of the central polypite or from
certain points on the edge of the disc; and this budding, unless it
occurs equally on all sides, which it does not and is unlikely to do,
must tend to destroy the balance of the disc, and to make its attitude
less changeable. In other cases the growth of a large process [a
much-developed tentacle] from the edge of the disc on one side, as in
_Steenstrupia_, Fig. 257, constitutes a similar modification, and a
cause of further modification. The animal is no longer divisible into
any two quite similar halves, except those formed by a plane passing
through the process; and unless the process is of the same specific
gravity as the disc, it must tend towards either the lowest or the
highest point, and must so serve to increase the bilateralness, by
keeping the two sides of the disc similarly conditioned while the
top and bottom are differently conditioned. Fig. 258 represents the
underside of another _Medusa_, in which a more decided bilateralness
is produced by the presence of two such processes. Among the simple
free-swimming _Actinozoa_, occur like deviations from radial symmetry,
along with like motions through the water in bilateral attitudes.
Of this a _Cydippe_ is a familiar example. Though radial in some of
its characters, as in the distribution of its meridional bands of
locomotive paddles with their accompanying canals, this creature
has a two-sided distribution of tentacles and various other parts,
corresponding with its two-sided attitude in moving through the water.
And in other genera of this group, as in _Cestum_, _Eurhamphæa_,
and _Callianira_, that almost equal distribution of parts which
characterizes the _Beroe_ is quite lost.
Here seems a fit place to meet the objection which some may feel to
this and other such illustrations, that they amount very much to
physical truisms. If the parts of a _Medusa_ are disposed in radial
symmetry round the axis of motion through the water, there will of
course be no means of maintaining one part of its edge uppermost
more than another; and the equality of conditions may be ascribed
to the radiateness, as much as the radiateness to the equality of
conditions. Conversely, when the parts are not radially arranged
around the axis of motion, they must gravitate towards some one
attitude, implying a balance on the two sides of a vertical plane--a
bilateralness; and the two-sided conditions so necessitated, may be
as much ascribed to the bilateralness as the bilateralness to the
two-sided conditions. Doubtless the form and the conditions are, in
the way alleged, necessary correlates; and in so far as it asserts
this, the objection harmonizes with the argument. To the difficulty
which it at the same time raises by the implied question--Why make
the form the result of the conditions, rather than the conditions
the result of the form? the reply is this:--The radial type, both as
being the least differentiated type and as being the most obviously
related to lower types, must be taken as antecedent to the bilateral
type. The individual variations which incidental circumstances produce
in the radial type, will not cause divergence of a species from the
radial type, unless such variations give advantages to the individuals
displaying them; which there is no reason to suppose they will always
do. Those occasional deviations from the radial type, which the law of
the instability of the homogeneous warrants us in expecting to take
place, will, however, in some cases be beneficial; and will then be
likely to establish themselves. Such deviations must tend to destroy
the original indefiniteness and variability of attitude--must cause
gravitation towards an habitual attitude. And gravitation towards an
habitual attitude having once commenced, will continually increase,
where increase of it is not negatived by adverse agencies: each further
degree of bilateralness rendering more decided the actions that conduce
to bilateralness. If this reply be thought insufficient, it may be
enforced by the further one, that as, among plants, the incident forces
are the antecedents and the forms the consequents (changes of forces
being in many cases visibly followed by changes of forms) we are
warranted in concluding that the like order of cause and effect holds
among animals.[44]
§ 247. Keeping to the same type but passing to a higher degree of
composition, we meet more complex and varied illustrations of the same
general laws. In the compound _Cœlenterata_, presenting clusters
of individuals which are severally homologous with the solitary
individuals last dealt with, we have to note both the shapes of the
individuals thus united, and the shapes of the aggregates made up of
them.
[Illustration: Figs. 149, 150.]
Such of the fixed _Hydrozoa_ and _Actinozoa_ as form branched
societies, continue radial; both because their varied attitudes do not
expose them to appreciable differences in their relations to those
surrounding actions which chiefly concern them (the actions of prey),
and because such differences, even if they were appreciable, would
be so averaged in their effects on the dissimilarly-placed members
of each group as to be neutralized in the race. Among the tree-like
coral-polypedoms, as well as in such ramified assemblages of simpler
polypes as are shown in Figs. 149, 150, we have, indeed, cases in many
respects parallel to the cases of scattered flowers (§ 233), which
though placed laterally remain radial, because no differentiating
agency can act uniformly on all of them. Meanwhile, in the groups which
these united individuals compose, we see the shapes of plants further
simulated under a further parallelism of conditions. The attached ends
differ from the free ends as they do in plants; and the regular or
irregular branches obviously stand to environing actions in relations
analogous to those in which the branches of plants stand.
The members of those compound _Cœlenterata_ which move through the
water by their own actions, in attitudes that are approximately
constant, show us a more or less distinct two-sidedness. _Diphyes_,
Fig. 259, furnishes an example. Each of the largely-developed and
modified polypites forming its swimming sacs is bilateral, in
correspondence with the bilateralness of its conditions; and in each of
the appended polypites the insertion of the solitary tentacle produces
a kindred divergence from the primitive radial type. The aggregate,
too, which here very much subordinates its members, exhibits the same
conformity of structure to circumstances. It admits of symmetrical
bisection by a plane passing through its two contractile sacs, or
nectocalyces, but not by any other plane; and the plane which thus
symmetrically bisects it, is the vertical plane on the two sides of
which its parts are similarly conditioned as it propels itself through
the water.
[Illustration: Fig. 259.]
Another group of the oceanic _Hydrozoa_, the _Physophoridæ_, furnishes
interesting evidence--not so much in respect of the forms of the united
individuals, which we may pass over, as in respect of the forms of the
aggregates. Some of these are without swimming organs, and have their
parts suspended from air-vessels which habitually float on the surface
of the water. Hence the distribution of their parts is asymmetrical.
The _Physalia_, Fig. 152, is an example. Here the relations of the
integrated group of individuals to the environment are indefinite; and
there is thus no agency tending to change that comparatively irregular
mode of growth which is probably derived from a primordial type of the
branched _Hydrozoa_.
[Illustration: Fig. 152.]
So various are the modes of union among the compound _Cœlenterata_,
that it is out of the question to deal with them all. Even did
space permit, it would be impracticable for any one but a professed
naturalist, to trace throughout this group the relations between
shapes and conditions of existence. The above must be taken simply as a
few of the most significant and easily-interpretable cases.
§ 248. In the sub-kingdoms _Polyzoa_ and _Tunicata_ we meet with
examples not wholly unlike the foregoing. Among the types assembled
under these names there are simple individuals or aggregates of the
second order, and societies or tertiary aggregates produced by their
union. The relations of forms to forces have to be traced in both.
Solitary Ascidians, fixed or floating, carry on an inactive and
indefinite converse with the actions in the environment. Without
power to move about vivaciously, and unable to catch any prey but
that contained in the currents of water they absorb and expel, these
creatures are not exposed to sets of forces which are equal on two
or more sides; and their shapes consequently remain vague. Though
internally their parts have a partially-symmetrical arrangement, due
to their derivation, yet they are substantially unsymmetrical in that
part of the body which is concerned with the environment. Fig. 156 is
an example.[45] Among the composite Ascidians, floating and fixed,
the shape of the aggregate, partly determined by the habitual mode of
gemmation and partly by the surrounding conditions in each case, is in
great measure indefinite. We can say no more about it than that it is
not obviously at variance with the laws alleged.
Evidence of a more positive kind occurs among those compound
_Molluscoida_ which are most like the compound _Cœlenterata_ in
their modes of union--the _Polyzoa_. Many of these form groups that
are more or less irregular--spreading as films over solid surfaces,
combining into seaweed-like fronds, budding out from creeping stolons,
or growing up into tree-shaped societies; and besides aggregating
irregularly they are irregularly placed on surfaces inclined in all
directions. Merely noting that this asymmetrical distribution of the
united individuals is explained by the absence of definiteness in the
relations of the aggregate to incident forces, it concerns us chiefly
to observe that the united individuals severally exemplify the same
truth as do similarly-united individuals among the _Cœlenterata_.
Averaging the members of each society, the ciliated tentacles they
protrude are similarly related to prey on all sides; and therefore
remain the same on all sides. This distribution of tentacles is not,
however, without exception. Among the fresh-water _Polyzoa_ there
are some genera, as _Plumatella_ and _Crystatella_, in which the
arrangement of these parts is very decidedly bilateral. Some species of
them show us such relations of the individuals to one another and to
their surface of attachment, as give a clue to the modification; but in
other species the meaning of this deviation from the radial type is not
obvious.
§ 249. In the _Platyhelminthes_ good examples of the connexions between
forms and forces occur. The _Planaria_ exemplifies the single bilateral
symmetry which, even in very inferior forms, accompanies the habit of
moving in one direction over a solid surface. Humbly organized as are
these creatures and their allies the _Nemertidæ_, we see in them, just
as clearly as in the highest animals, that where the movements subject
the body to different forces at its two ends, different forces on its
under and upper surfaces, and like forces along its two sides, there
arises a corresponding form, unlike at its extremities, unlike above
and below, but having its two sides alike.
The _Echinodermata_ furnish us with instructive
illustrations--instructive because among types that are nearly allied,
we meet with wide deviations of form answering to marked contrasts in
the relations to the environment. The facts fall into four groups.
The _Crinoidea_, once so abundant and now so rare, present a radial
symmetry answering to an incidence of forces that are equal on all
sides. In the general attitudes of their parts towards surrounding
actions, they are like uniaxial plants or like polypes; and show, as
those do, marked differences between the attached ends and the free
ends, along with even distributions of parts all round their axes.
In the _Ophiuridea_, and in the Star-fishes, we have radial symmetry
co-existing with very different habits; but habits which nevertheless
account for the maintenance of the form. Holding on to rocks and weeds
by its simple or branched arms, or by the suckers borne on the under
surface of its rays, one of these creatures moves about not always
with one side foremost, but with any side foremost. Consequently,
averaging its movements, its arms or rays are equally affected, and
therefore remain the same on all sides. On watching the ways of the
common Sea-urchin, we are similarly furnished with an explanation of
its spherical, or rather its spheroidal, figure. Here the habit is
not to move over any one approximately-flat surface; but the habit is
to hold on by several surfaces on different sides at the same time.
Frequenting crevices and the interstices among stones and weeds, the
Sea-urchin protrudes the suckers arranged in meridional bands over
its shell, laying hold of objects now on this side and now on that,
now above and now below: the result being that it does not move in
all directions over one plane but in all directions through space.
Hence the approach in general form towards spherical symmetry--an
approach which is, however, restrained by the relations of the parts
to the mouth and vent: the conditions not being exactly the same at
the two poles as at other parts of the surface. Still more significant
is that deviation from this shape which occurs among such of the
_Echinidea_ as have habitats of a different kind, and consequently,
different habits. The genera _Echinocyamus_, _Spatangus_, _Brissus_,
and _Amphidotus_, diverge markedly towards a bilateral structure. These
creatures are found not on rocky shores but on flat sea-bottoms, and
some of them only on bottoms of sand or mud. Here, there is none of
that distribution of surfaces on all sides which makes the spheroidal
form congruous with the conditions. Having to move about over an
approximately-horizontal plane, any deviation of structure arising
accidentally which leads to one side being kept always foremost, will
be an advantage: greater fitness to function becoming possible in
proportion as function becomes fixed. Survival of the fittest will
therefore tend to establish, under such conditions, a form that keeps
the same part in advance--a form in which, consequently, the original
radial symmetry diverges more and more towards bilateral symmetry.
§ 250. Very definite and comparatively uniform, are the relations
between shapes and circumstances among the _Annulosa_: including under
that title the _Annelida_ and the _Arthropoda_. The agreements and the
disagreements are equally instructive.
At one time or other of its life, if not throughout its life, every
annulose animal is locomotive; and its temporary or permanent
locomotion, being carried on with one end habitually foremost and one
surface habitually uppermost, it fulfils those conditions under which
bilateral symmetry arises. Accordingly, bilateral symmetry is traceable
throughout the whole of this sub-kingdom. Traceable, we must say,
because, though it is extremely conspicuous in the immense majority
of annulose types, it is to a considerable extent obscured where
obscuration is to be expected. The embryos of the _Tubicolæ_, after
swimming about a while, settle down and build themselves tubes, from
which they protrude their heads; and in them, or in some of them, the
bilateral symmetry is disguised by the development of head-appendages
in an all-sided manner. The tentacles of _Terebella_ are distributed
much in the same way as those of a polype. The breathing organs in
_Sabella unispira_, Fig. 260, do not correspond on opposite sides of
a median plane. Even here, however, the body retains its primitive
bilateralness; and it is further to be remarked that this loss of
bilateralness in the external appendages, does not occur where the
relations to external conditions continue bilateral: witness the
_Serpula_, Fig. 261, which has its respiratory tufts arranged in a
two-sided way, under the two-sided conditions involved by the habitual
position of its tube.
[Illustration: Figs. 260–261.]
The community of symmetry among the higher _Annulosa_, has an
unobserved significance. That Flies, Beetles, Lobsters, Centipedes,
Spiders, Mites, have in common the characters, that the end which moves
in advance differs from the hinder end, that the upper surface differs
from the under surface, and that the two sides are alike, is a truth
received as a matter of course. After all that has been said above,
however, it will be seen to have a meaning not to be overlooked; since
it supplies a million-fold illustration of the laws which have been set
forth. It is needless to give diagrams. Every reader can call to mind
the unity indicated.
[Illustration: Figs. 263–270.]
While, however, annulose animals repeat so uniformly these traits of
structure, there are certain other traits in which they are variously
contrasted; and their contrasts have to be here noted, as serving
further to build up the general argument. In them we see the stages
through which bilateral symmetry becomes gradually more marked, as
the conditions it responds to become more decided. A common Earth-worm
may be instanced as a member of this sub-kingdom that is among the
least-conspicuously bilateral. Though internally its parts have a
two-sided arrangement; and though the positions of its orifices give
it an external two-sidedness, at the same time that they establish
a difference between the two ends; yet its two-sidedness is not
strongly-marked. The form deviates but little from what we have
distinguished as triple bilateral symmetry: if the creature is cut
across the middle, the head and tail ends are very much alike; if
cut in two along its axis by a horizontal plane, the under and upper
halves are very much alike, externally if not internally; and if cut
in two along its axis by a vertical plane, the two sides are quite
alike. Figs. 263 and 264 will make this clear. Such creatures as
the _Julus_ and the Centipede, may be taken as showing a transition
to double bilateral symmetry. Besides being divisible into exactly
similar halves by a vertical plane passing through its axis, one of
these animals may be bisected transversely into parts that differ only
slightly; but if cut in two by a horizontal plane passing through its
axis, the under and upper halves are decidedly unlike. Figs. 265, 266,
exhibit these traits. Among the isopodous crustaceans, the departure
from these low types of symmetry is more marked. As shown in Figs. 267
and 268, the contrast between the upper and under parts is greater,
and the head and tail ends differ more obviously. In all the higher
_Arthropoda_, the unlikeness between the front half and the hind half
has become conspicuous. There is in them single bilateral symmetry of
so pronounced a kind, that no other resemblance is suggested than that
between the two sides. By Figs. 269 and 270, representing a decapodous
crustacean divided longitudinally and transversely, this truth is made
manifest. On calling to mind the habits of the creatures here drawn and
described, it will be seen that they explain these forms. The incidence
of forces is the same all around the Earth-worm as it burrows through
the compact ground. The Centipede, creeping amid loose soil or _débris_
or beneath stones, insinuates itself between solid surfaces--the
interstices being mostly greater in one dimension than in others.
And all the higher _Annulosa_, moving about as they do over exposed
objects, have their dorsal and ventral parts as dissimilarly acted upon
as are their two ends.
[Illustration: Fig. 271.]
One other fact only respecting annulose animals needs to be noticed
under this head--the fact, namely, that they become unsymmetrical
where their parts are unsymmetrically related to the environment.
The common Hermit-crab serves as an instance. Here, in addition
to the unlikeness of the two sides implied by that curvature of
the body which fits the creature to the shell it inhabits, there
is an unlikeness due to the greater development of the limbs, and
especially the claws, on the outer side. As in the embryo of the
Hermit-crab the two sides are alike; and as both the embryo and
the ancestor lived in such a way, being free, that the conditions
were alike on the two sides; and as the embryo may be taken to
represent the type from which the Hermit-crab has been derived; we
have in this case evidence that a symmetrically-bilateral form has
been moulded into an unsymmetrically-bilateral form, by the action
of unsymmetrically-bilateral conditions. A further illustration
is supplied by _Bopyrus_, Fig. 271: a parasite which lives in the
branchial chamber of prawns, and whose habits similarly account for its
distorted shape.
§ 251. Among the _Mollusca_ we find more varied relations between
shapes and circumstances. Some of these relations are highly
instructive.
Mollusks of one order, the _Pteropoda_, swim in the sea much in
the same way that butterflies fly in the air, and have shapes not
altogether unlike those of butterflies. Fig. 272 represents one of
these creatures. That its bilaterally-symmetrical shape harmonizes with
its bilaterally-symmetrical conditions is sufficiently obvious.
[Illustration: Fig. 272.]
Among the _Lamellibranchiata_, we have diverse forms accompanying
diverse modes of life. Such of them as frequently move about, like
the fresh-water Mussel, have their two valves and the contained parts
alike on the opposite sides of a vertical plane: they are bilaterally
symmetrical in conformity with their mode of movement. The marine
Mussel, too, though habitually fixed, and though not usually so fixed
that its two valves are similarly conditioned, still retains that
bilateral symmetry which is characteristic of the order; and it does
this because in the species considered as a whole, the two valves
are not dissimilarly conditioned. If the positions of the various
individuals are averaged, it will be seen that the differentiating
actions neutralize one another. In certain other fixed Lamellibranchs,
however, there is a considerable deviation from bilateral symmetry;
and it is a deviation of the kind to be anticipated under the
circumstances. Where one valve is always downwards, or next to the
surface of attachment, while the other valve is always upwards, or next
to the environing water, we may expect to find the two valves become
unlike. This we do find: witness the Oyster. In the Oyster, too, we see
a further irregularity. There is a great indefiniteness of outline,
both in the shell and in the animal--an indefiniteness made manifest by
comparing different individuals. We have but to remember that growing
clustered together, as Oysters do, they must interfere with one another
in various ways and degrees, to see how the indeterminateness of form
and the variety of form are accounted for.
Among the Gasteropods modifications of a more definite kind occur. “In
all Mollusks,” says Professor Huxley, “the axis of the body is at first
straight, and its parts are arranged symmetrically with regard to a
longitudinal vertical plane, just as in a vertebrate or an articulate
embryo.” In some Gasteropods, as the _Chiton_, this bilateral
symmetry is retained--the relations of the body to surrounding actions
not being such as to disturb it. But in those more numerous types
which have spiral shells, there is a marked deviation from bilateral
symmetry, as might be expected. “This asymmetrical over-development
never affects the head or foot of the mollusk”: only those parts
which, by inclosure in a shell, are protected from environing actions,
lose their bilateralness; while the external parts, subjected by the
movements of the creatures to bilateral conditions, remain bilateral.
Here, however, a difficulty meets us. Why is it that the naked
Gasteropods, such as our common slugs, deviate from bilateral symmetry,
though their modes of movement are those along with which complete
bilateral symmetry usually occurs? The reply is that their deviations
from bilateral symmetry are probably inherited, and that they are
maintained in such parts of their organization as are not exposed to
bilaterally-symmetrical conditions. There is reason to believe that the
naked Gasteropods are descended from Gasteropods which had shells: the
evidence being that the naked Gasteropods have shells during the early
stages of their development, and that some of them retain rudimentary
shells throughout life. Now the shelled Gasteropods deviate from
bilateral symmetry in the disposition of both the alimentary system
and the reproductive system. The naked Gasteropods, in losing their
shells, have lost that immense one-sided development of the alimentary
system which fitted them to their shells, and have acquired that
bilateral symmetry of external figure which fits them for their habits
of locomotion; but the reproductive system remains one-sided, because,
in respect to it, the relations to external conditions remain one-sided.
The Cephalopods show us bilaterally-symmetrical external forms along
with habits of movement through the water in two-sided attitudes. At
the same time, in the radial distribution of the arms, enabling one
of these creatures to take an all-sided grasp of its prey, we see how
readily upon one kind of symmetry there may be partially developed
another kind of symmetry, where the relations to conditions favour it.
§ 252. The _Vertebrata_ illustrate afresh the truths which we have
already traced among the _Annulosa_. Flying through the air, swimming
through the water, and running over the earth as vertebrate animals
do, in common with annulose animals, they are, in common with annulose
animals, different at their anterior and posterior ends, different
at their dorsal and ventral surfaces, but alike along their two
sides. This single bilateral symmetry remains constant under the
extremest modifications of form. Among fish we see it alike in the
horizontally-flattened Skate, in the vertically-flattened Bream, in the
almost-spherical _Diodon_, and in the greatly-elongated _Syngnathus_.
Among reptiles the Turtle, the Snake, and the Crocodile all display it.
And under the countless modifications of structure displayed by birds
and mammals, it remains conspicuous.
[Illustration: Figs. 273–280.]
A less obvious fact which it concerns us to note among the
_Vertebrata_, parallel to one which we noted among the _Annulosa_, is
that whereas the lower vertebrate forms deviate but little from triple
bilateral symmetry, the deviation becomes great as we ascend. Figs.
273 and 274 show how, besides being divisible into similar halves by
a vertical plane passing through its axis, a Fish is divisible into
halves that are not very dissimilar by a horizontal plane passing
through its axis, and also into other not very dissimilar halves by
a plane cutting it transversely. If, as shown in Figs. 275 and 276,
analogous sections be made of a superior Reptile, the divided parts
differ more decidedly. When a Mammal and a Bird are treated in the
same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts
marked off by the dividing planes are unlike in far greater degrees. On
considering the mechanical converse between organisms of these several
types and their environments--on remembering that the fish habitually
moves through a homogeneous medium of nearly the same specific gravity
as itself, that the terrestrial reptile either crawls on the surface or
raises itself very incompletely above it, that the more active mammal,
having its supporting parts more fully developed, thereby has the under
half of its body made more different from the upper half, and that the
bird is subject by its mode of life to yet another set of actions and
reactions; we shall see that these facts are quite congruous with the
general doctrine, and furnish further support to it.
One other significant piece of evidence must be named. Among the
_Annulosa_ we found unsymmetrical bilateralness in creatures having
habits exposing them to unlike conditions on their two sides; and
among the _Vertebrata_ we find parallel cases. They are presented
by the _Pleuronectidæ_--the order of distorted flat fishes to which
the Sole and the Flounder belong. On the hypothesis of evolution, we
must conclude that fishes of this order have arisen from an ordinary
bilaterally-symmetrical type of fish, which, feeding at the bottom of
the sea, gained some advantage by placing itself with one of its sides
downwards, instead of maintaining the vertical attitude. Besides the
general reason there are special reasons for concluding this. In the
first place, the young Sole or Flounder is bilaterally symmetrical--has
its eyes on opposite sides of its head and swims in the usual way. In
the second place, the metamorphosis which produces the unsymmetrical
structure sometimes does not take place--there are abnormal Flounders
that swim vertically, like other fishes. In the third place, the
transition from the symmetrical structure to the unsymmetrical
structure may be traced. Almost incredible though it seems, one of the
eyes is transferred from the underside of the head to the upper side:
the transfer being effected by a distorted development of the cranial
bones--atrophy of some and hypertrophy of others, along with a general
twist. This metamorphosis furnishes several remarkable illustrations
of the way in which forms become moulded into harmony with incident
forces. For besides the divergence from bilateral symmetry involved
by presence of both eyes upon the upper side, there is a further
divergence from bilateral symmetry involved by differentiation of the
two sides in respect to the contours of their surfaces and the sizes of
their fins. And then, what is still more significant, there is a near
approach to likeness between the halves that were originally unlike,
but are, under the new circumstances, exposed to like conditions. The
body is divisible into similarly-shaped parts by a plane cutting it
along the side from head to tail: “the dorsal and ventral instead of
the lateral halves become symmetrical in outline and are equipoised.”
§ 253. Thus, little as there seems in common between the shapes of
plants and the shapes of animals, we yet find, on analysis, that the
same general truths are displayed by both. The one ultimate principle
that in any organism equal amounts of growth take place in those
directions in which the incident forces are equal, serves as a key to
the phenomena of morphological differentiation. By it we are furnished
with interpretations of those likenesses and unlikenesses of parts,
which are exhibited in the several kinds of symmetry; and when we take
into account inherited effects, wrought under ancestral conditions
contrasted in various ways with present conditions, we are enabled to
comprehend, in a general way, the actions by which animals have been
moulded into the shapes they possess.
To fill up the outline of the argument, so as to make it correspond
throughout with the argument respecting vegetal forms, it would be
proper here to devote a chapter to the differentiations of those
homologous segments out of which animals of certain types are composed.
Though, among most animals of the third degree of composition, such
as the rooted _Hydrozoa_, the _Polyzoa_, and the _Ascidioida_, the
united individuals are not reduced to the condition of segments of a
composite individual, and do not display any marked differentiations;
yet there are some animals in which such subordinations, and consequent
heterogeneities, occur. The oceanic _Hydrozoa_ form one group of them;
and we have seen reason to conclude that the _Annulosa_ form another
group. It is not worth while, however, to occupy space in detailing
these unlikenesses of homologous segments, and seeking specific
explanations of them. Among the oceanic _Hydrozoa_ they are extremely
varied; and the habits and derivations of these creatures are so little
known, that there are no adequate data for interpreting the forms of
the parts in terms of their relations to the environment. Conversely,
among the _Annulosa_ those differentiations of the homologous segments
which accompany their progressing integration, have so much in common,
and have general causes which are so obvious, that it is needless to
deal with them at any length. They are all explicable as due to the
exposure of different parts of the chain of segments to different sets
of actions and reactions: the most general contrast being that between
the anterior segments and the posterior segments, answering to the most
general contrast of conditions to which annulose animals subject their
segments; and the more special contrasts answering to the contrasts of
conditions entailed by their more special habits.
Were an exhaustive treatment of the subject practicable, there should
here, also, come a chapter devoted to the internal structures of
animals--meaning, more especially, the shapes and arrangements of the
viscera. The relations between forms and forces among these inclosed
parts are, however, mostly too obscure to allow of interpretation.
Protected as the viscera are in great measure from the incidence of
external forces, we are not likely to find much correspondence between
their distribution and the distribution of external forces. In this
case the influences, partly mechanical, partly physiological, which
the organs exercise on one another, become the chief causes of their
changes of figure and arrangement; and these influences are complex and
indefinite. One general fact may, indeed, be noted--the fact, namely,
that the divergence towards asymmetry which generally characterizes
the viscera, is marked among those of them which are most removed from
mechanical converse with the environment, but not so marked among
those of them which are less removed from such converse. Thus while,
throughout the _Vertebrata_, the alimentary system, with the exception
of its two extremities, is asymmetrically arranged, the respiratory
system, which occupies one end of the body, generally deviates but
little from bilateral symmetry, and the reproductive system, partly
occupying the other end of the body, is in the main bilaterally
symmetrical: such deviation from bilateral symmetry as occurs, being
found in its most interiorly-placed parts, the ovaries. Just indicating
these facts as having a certain significance, it will be best to leave
this part of the subject as too involved for detailed treatment.
Internal structures of one class, however, not included among
the viscera, admit of general interpretation--structures which,
though internal, are brought into tolerably-direct relations with
environing forces, and are therefore subordinate in their forms to
the distribution of those forces. These internal structures it will
be desirable to deal with at some length; both because they furnish
important illustrations enforcing the general argument, and because an
interpretation of them which we have seen reason to reject, cannot be
rejected without raising the demand for some other interpretation.